The present invention relates to a spread spectrum communication apparatus used in a spread spectrum communication system, a demodulator used in the communication apparatus, a surface acoustic wave element used in the demodulator, and surface acoustic wave parts used in the demodulator.
In recent years, a spread spectrum communication system (SS communication system) having strong noise resistivity and excellent secrecy and concealability has received attention as a communication system for civil use. In the SS communication system, carrier waves obtained by modulating information to be transmitted by a carrier signal are subjected to spread spectrum modulation (SS modulation) by use of a predetermined code series having a high chip rate to obtain a spread spectrum signal (SS signal) which is transmitted as a transmit signal. In this case, the code series may include a pseudo noise code series (PN code series) and a Barker code series. The SS modulation system may include a direct spread system (DS system) and a frequency hopping system (FH system).
In such an SS communication system, the receiver side requires a demodulator for demodulating the SS signal transmitted thereto. For example, in the case where the SS modulation is based on the DS system by use of the PN code series, the receiver side uses the same PN code series as that on the transmitter side for demodulation. At this time demodulators are roughly divided into demodulators using ICS and demodulators using surface acoustic wave elements. A surface acoustic wave element used in a demodulator has become an object of attention since the demodulator can be fabricated at a low cost and with a simple construction by using a photolithography technique for formation of the surface acoustic wave element.
Surface acoustic wave elements can be classified into surface acoustic wave matched filters and surface acoustic wave convolvers from the structural aspect. In the surface acoustic wave convolver, it is possible to select a PN code series which is used for modulation. Therefore, the surface acoustic wave convolver is suitable for use in applications in which secrecy and concealability are especially required. In the surface acoustic wave matched filter, a PN code series used for modulation is fixed but a peripheral circuit can correspondingly be formed with a simple construction, thereby providing the whole system at a low cost. Therefore, the surface acoustic wave matched filter has become an object of attention as a demodulator used in a small-scale SS communication system, for example, a private wireless LAN.
The construction of a conventional demodulator for a DS system using a surface acoustic wave matched filter is shown by FIG. 11 in block diagram. In the figure, reference numeral 61 denotes a surface acoustic wave matched filter inputted with an SS signal s for outputting a correlation signal m, numeral 62 denotes a surface acoustic wave delay line for delaying the correlation signal m from the surface acoustic wave matched filter 61 by one period, numeral 63 denotes an integrating circuit for integrating the correlation signal m from the surface acoustic wave matched filter 61 and a correlation signal n from the surface acoustic wave delay line 62 subjected to the delay of one period, numeral 64 denotes an amplifier for amplifying the correlation signal m from the surface acoustic wave matched filter 61, and the symbols L1 and L2 denote signal lines.
The operation of the demodulator shown in FIG. 11 will now be explained briefly. An SS signal s inputted to the surface acoustic wave matched filter 61 is converted by the surface acoustic wave matched filter 61 into a correlation signal m which is in turn divided into two systems including the lines L1 and L2. The correlation signal m on the line L1 is inputted directly to the integrating circuit 63. The correlation signal m on the other line L2 is inputted to the surface acoustic wave delay line 62 through the amplifier 64 so that it is inputted to the integrating circuit 63 as a correlation signal n delayed by one period. The integrating circuit 63 integrates the correlation signal m and the one-period delayed signal n to obtain a demodulated signal.
FIG. 12A is a pattern diagram showing the surface acoustic wave matched filter in the demodulator shown in FIG. 11. In FIG. 12A, reference numeral 71 denotes a piezoelectric substrate made of quartz crystal, LiNbO.sub.3 or the like, numeral 72 denotes a signal input electrode, numeral 73 denotes an output encoding electrode, and numeral 74 denotes an acoustic material member for absorbing unnecessary surface acoustic waves. Next, an explanation of operation will be made. The signal input electrode 72 has a comb form for converting an electric signal into surface acoustic waves. The output encoding electrode 73 is separated from the electrode 72 by a predetermined interval and converts the surface acoustic waves into an electric signal. The electrodes 72 and 73 are provided on the piezoelectric substrate 71 to form a surface acoustic wave matched filter. In the case where a PN code series of n bits is used, the output encoding electrode 73 has n comb-like electrode pairs corresponding to the n-bit PN code series and the comb-like electrode pairs are formed at intervals corresponding to the chip rate. In this case, the number of pairs of electrodes (or electrode fingers) in a comb-like electrode pair is 1 (one). For the purpose of absorbing unnecessary surface acoustic waves, the acoustic material members 74 are formed outside of the input and output electrodes 72 and 73, as required. In this case, the signal input electrode 72 and the output encoding electrode 73 may be reversed, that is, the signal input electrode 72 and the output encoding electrode 73 may be used as an output electrode and an input electrode, respectively.
FIG. 12B is a pattern diagram showing the surface acoustic wave delay line in the demodulator shown in FIG. 11. In FIG. 12B, reference numeral 75 denotes a piezoelectric substrate made of quartz crystal, LiNbO.sub.3 or the like, numeral 76 denotes a signal input electrode, numeral 77 denotes a signal output electrode, and numeral 78 denotes acoustic material members for absorbing unnecessary surface acoustic waves. Next, an explanation of operation will be made. The signal input electride 76 has a comb form for converting an electric signal into surface acoustic waves. The signal output electrode 77 also has a comb form and is separated from the electrode 76 by an interval corresponding to one period T of a signal to be received and demodulated. The electrode 77 converts the surface acoustic waves into an electric signal. The electrodes 76 and 77 are provided on the piezoelectric substrate 75 to form a surface acoustic wave delay line. For the purpose of absorbing unnecessary surface acoustic waves, the acoustic material members 74 are formed outside of the input and output electrodes 76 and 77, as required.
A demodulator using such a surface acoustic wave matched filter performs demodulation by use of two polarities (for example, 0 phase and .pi. phase) which the surface acoustic wave matched filter takes. The modulation system corresponds to binary phase shift keying system (BPSK system).
Though the transmission rate of information in a wireless LAN or the like is as high as possible, the transmission rate in an SS communication is restricted by the band width of the SS communication system itself and the PN code series that is used. Namely, it is required that the transmission rate should be lower than a value obtained by dividing the band width by 2n, wherein n is the number of bits in the PN code series. From the aspect of transmission rate, therefore, it is preferable that the number of bits in the PN code series is made small. However, if the number of bits in the PN code series is too small, there is an inconvenience in that the secrecy or concealability of the SS communication system deteriorate or a sufficient processing gain is not obtained. Therefore, a method in which the modulation system itself is transformed to a four-phase or quadri-phase shift keying system (QPSK system) that is capable of a transmission rate substantially twice as high as that in the BPSK system without changing the number of bits in the PN code series might be considered.
However, the QPSK system requires the discrimination of four states that are different in phase by 90 degrees though the discrimination of two states different in phase by 180 degrees (or 0 phase and .pi. phase) suffices for the BPSK system. The conventional demodulator has a problem that it can cope with the BPSK system but cannot cope with the QPSK system.